a. Field of the Invention
The present invention relates to an optical system adapted for image forming apparatus in which image is formed by exposing an image of original to photosensitive body, more particularly relates to a variable magnification optical mechanism or variation optical mechanism for forming copy image of an original at variable magnification including magnification in negative direction, namely minification.
B. Description of the Prior Art
In the art of image forming apparatus provided with variable magnification optical mechanism such as electrostatic copying apparatus, there are known two types of exposure system.
One is of the type in which the entire area of original is exposed to the photosensitive body at a time, which is called "full illumination type".
The other is of the type so called "scanning type" in which the original is successively exposed from one edge to the other at every strip like section having the same width. Practically the latter is more widely employed than the former.
In case of the latter, it is known that a preliminary operation of the optical system is required which is usually called "pre-running". This is because at the starting time of movable optical members such as mirror and original table, there occur vibrations that give adverse effects to the image, for example, make the produced image unclear.
To prevent such adverse effect caused by vibration of mirror or original table at their starting time, they are preliminarily moved in actual exposure step by a predetermined distance enough to have them reached a given operational speed. This additional running of movable part or members is called "pre-running". This is troublesome and unpreferable at any rate. Notwithstanding, however, the scanning type of apparatus has been widely employed and preferred.
This is because it has various advantages adequately enough to compensate for the above mentioned disadvantage. For example, it permits miniaturization of optical system assembly and use of small light source of exposure.
But, if a variation optical mechanism, for example, to minify the image of original is incorporated into such scanning type of optical system, there arises another problem. The problem is that those copy images are produced in which image of the fore edge portion is missing. This is caused by the fact that when a minified copy image is formed, the prerunning of the optical system is prematurely finished relative to the photosensitive body that moves always at the same speed given for copying at 1:1 magnification, and, therefore, imagewise exposure begins already in the course of pre-running.
To solve the problem, it has known and used practically that photosensitive sheet (for FAX type copying apparatus) or transfer sheet (for transfer type copying apparatus) is fed and conveyed with a suitable time delay enough to obtain coincidence. However, there are arranged along the path of photosensitive copy sheet or transfer sheet various elements and members such as control means for sheet feeding and conveying device, control means for transfer sheet separator and blocking detector at fixing station. Due to this fact, control of the necessary time delay mentioned above becomes very complicate and troublesome.